The present invention relates generally to the detection of an abnormality of a sensor, and more particularly to an apparatus for detecting failures of a sensor such as a disconnection of a pickup coil, and is applicable particularly, but not exclusively, to a control system for use in a motor vehicle.
Generally, control systems such as an anti-skid control apparatus for use in a motor vehicle are arranged to control the vehicle on the basis of the output signals of sensors such as a rotational speed sensor. Failures such as disconnection of the rotational speed sensor result in difficulty in ensuring appropriate vehicle control. In order to solve this problem, the control system is provided with an apparatus for detecting failures of the rotational speed sensor. One example of such an apparatus is illustrated in FIG. 6 wherein a monitoring circuit 5 for detection of disconnection is provided, which monitors a signal in a signal-carrying line 4 coupled through a connector 1 to an output line 3 of a pickup coil 2 of a sensor. The monitoring circuit 5, for example, is arranged as shown in FIG. 7. Another known arrangement is disclosed in Japanese patent application No. 48-11189, the teachings of which involve the detection of a short-circuit of a pickup coil of a sensor in addition to disconnection thereof.
Such prior art systems have been disadvantageous, however, in that the connector presents a resistance against the output current of the pickup coil due to an oxide film that accumulates over time, and the attenuation of the output current results in the fact that a difficulty is encountered to meet the requirements imposed in high precision detection of failures of a rotational speed sensor such as disconnection.